The present disclosure relates generally o well logging techniques and, more particularly, to a neutron-gamma density measurement that accounts for both liquid-filled and gas-filled formations.
This section is intended to introduce the reader to various aspects of art that may he related to various aspects of the present techniques, which are described and/or claimed below. This discussion is believed to be helpful in providing the reader with background information to facilitate a better understanding of the various aspects of the present disclosure. Accordingly, it should be understood that these statements are to be read in this light, and not as admissions of prior art.
To determine the composition and porosity of a subterranean formation, several simultaneous measurements are obtained, namely hydrogen index and formation density. Hydrogen index, which corresponds to the hydrogen content of the formation, can be determined based on neutron transport through the formation, since neutron flux through a formation attenuated with distance from a neutron source depends strongly on the hydrogen content of the formation. When neutrons are emitted into the formation from neutron source in a downhole tool, and subsequently detected by a neutron detector of the downhole tool after scattering in he formation, the hydrogen index of the formation may be determined.
Hydrogen index generally relates to the porosity of the formation because pore spaces of the formation may be filled with some amount of hydrogen. For liquid-filled pores, the hydrogen index may correspond to the porosity of the formation in a properly calibrated tool. On the other hand, when the pore spaces of the formation are filled with gas rather than liquid, the hydrogen index measurement can he misleading. Indeed, the pores of a gas-filled formation may hold less hydrogen than the pores of a liquid-filled formation of the same porosity. In other words, gas-filled and liquid-filled formations of the same porosity have different densities. Thus, the formation density measurement may be used to determine the degree to which a formation is gas-filled or liquid-filled, revealing the proper porosity of the formation.
While the hydrogen index measurement may involve neutron transport, the formation density measurement involves the scattering of gamma-rays through the formation. Conventionally, obtaining a gamma density involves irradiating the formation with gamma-rays using a radioisotopic source (e.g., 137Cs or 241AmBe). These gamma-rays may Compton scatter from the electrons present in the formation before being detected by a gamma-ray detector spaced some distance from the gamma-ray source. Since the electron concentration is proportional to the atomic number of the elements, and the degree to which the gamma-rays Compton scatter and return to the gamma-ray detector relates to the electron concentration, the density of the formation may be detected using this technique.
The use of radioisotopic sources such as 137Cs or 241AmBe may be undesirable in a downhole tool. Among other things, such radioisotopic sources may present an environmental concern and may involve special handling requirements. Additionally, any gamma-ray source that is used in a cased-hole density measurement may emit gamma-rays that are attenuated strongly by the casing material, resulting in much fewer gamma-rays reaching the formation than otherwise. This gamma-ray attenuation may cause a cased-hole gamma-gamma density measurement impossible or inaccurate.
Some techniques have been developed to generate gamma-rays for a formation density measurement without using any radioisotopic gamma-ray sources. Instead, gamma-rays for a formation density measurement may be created when neutrons, emitted by an electronic neutron generator, inelastically scatter off certain elements in the formation. Such a formation density measurement may be referred to as a neutron-gamma density (NGD) measurement, as distinguished from the conventional gamma-gamma density (GGD) measurements that rely on radioisotopic gamma-ray sources. In an NGD measurement, fast neutrons may pass through borehole casing material to reach the formation, where these neutrons may inelastically scatter on oxygen and certain other atoms in the formation. This inelastic scattering produces high-energy gamma-rays that can be detected by the gamma-ray detectors in the tool. Existing NGD techniques provide accurate results in the case of liquid-filled formations. However, in the presence of gas in the formation, the density reading according to existing NGD techniques tends to deviate from the real formation density.